The bonding properties of the model complexes Fe(CO)2L(η4-butadiene) and Fe(CO)2L(η4-enone) with L = CO, PH3, and P(OH)3 were studied. The characteristics of the frontier orbitals of the Fe(CO)2L fragments determine their interaction with the butadiene and enone ligands completing the metal coordination sphere. The frontier orbitals of butadiene are stabilized when one CH2 is replaced by the more electronegative oxygen atom. The staggered conformation is preferred by the Fe(CO)3(η4-enone) complexes. The most stable of the three possible staggered conformations of Fe(CO)2L(η4-enone) is the one in which the phosphorus ligand is approximately trans to the central C-C bond of the enone. The strongest interaction between the enone and Fe(CO)2L is back-donation from the HOMO of the metal fragment to the empty π3 orbital of the enone, more electrons thus being donated than received by the metal. Electron density is mainly lost by the oxygen atom and gained by the terminal carbon atom (C4) of the enone. The results were used to interpret the isomer distribution in Fe(CO)2L(η4-enone) and the electronic effects in Fe(CO)2L(η4-benzylideneacetone) complexes where L is CO, phosphine, and phosphite. © 1990 American Chemical Society.9410601067